The ABCA4 protein (then called a “rim protein”) was first identified in 1978 in the rims and incisures of rod photoreceptors. The corresponding gene, ABCA4 , was cloned in 1997, and variants were identified as the cause of autosomal recessive Stargardt disease (STGD1). Over the next two decades, variation in ABCA4 has been attributed to phenotypes other than the classically defined STGD1 or fundus flavimaculatus, ranging from early onset and fast progressing cone-rod dystrophy and retinitis pigmentosa-like phenotypes to very late onset cases of mostly mild disease sometimes resembling, and confused with, age-related macular degeneration. Similarly, analysis of the ABCA4 locus uncovered a trove of genetic information, including >1200 disease-causing mutations of varying severity, and of all types – missense, nonsense, small deletions/insertions, and splicing affecting variants, of which many are located deep-intronic. Altogether, this has greatly expanded our understanding of complexity not only of the diseases caused by ABCA4 mutations, but of all Mendelian diseases in general. This review provides an in depth assessment of the cumulative knowledge of ABCA4-associated retinopathy – clinical manifestations, genetic complexity, pathophysiology as well as current and proposed therapeutic approaches.

Background Variation in the ABCA4 gene is causal for, or associated with, a wide range of phenotypes from early onset Mendelian retinal dystrophies to late-onset complex disorders such as age-related macular degeneration (AMD). Despite substantial progress in determining the causal genetic variation, even complete sequencing of the entire open reading frame and splice sites of ABCA4 identifies biallelic mutations in only 60%–70% of cases; 20%–25% remain with one mutation and no mutations are found in 10%–15% of cases with clinically confirmed ABCA4 disease. This study was designed to identify missing causal variants specifically in monoallelic cases of ABCA4 disease. Methods Direct sequencing and analysis were performed in a large familial ABCA4 disease cohort of predominately European descent (n=643). Patient phenotypes were assessed from clinical and retinal imaging data. Results We determined that a hypomorphic ABCA4 variant c.5603A>T (p.Asn1868Ile), previously considered benign due to high minor allele frequency (MAF) (~7%) in the general population, accounts for 10% of the disease, >50% of the missing causal alleles in monoallelic cases, ~80% of late-onset cases and distinguishes ABCA4 disease from AMD. It results in a distinct clinical phenotype characterised by late-onset of symptoms (4th decade) and foveal sparing (85%). Intragenic modifying effects involving this variant and another, c.2588G>C (p.Gly863Ala) allele, were also identified. Conclusions These findings substantiate the causality of frequent missense variants and their phenotypic outcomes as a significant contribution to ABCA4 disease, particularly the late-onset phenotype, and its clinical variation. They also suggest a significant revision of diagnostic screening and assessment of ABCA4 variation in aetiology of retinal diseases.

Sequence analysis of the coding regions and splice site sequences in inherited retinal diseases is not able to uncover ∼40% of the causal variants. Whole-genome sequencing can identify most of the non-coding variants, but their interpretation is still very challenging, in particular when the relevant gene is expressed in a tissue-specific manner. Deep-intronic variants in ABCA4 have been associated with autosomal-recessive Stargardt disease (STGD1), but the exact pathogenic mechanism is unknown. By generating photoreceptor precursor cells (PPCs) from fibroblasts obtained from individuals with STGD1, we demonstrated that two neighboring deep-intronic ABCA4 variants (c.4539+2001G>A and c.4539+2028C>T) result in a retina-specific 345-nt pseudoexon insertion (predicted protein change: p.Arg1514Leufs36), likely due to the creation of exonic enhancers. Administration of antisense oligonucleotides (AONs) targeting the 345-nt pseudoexon can significantly rescue the splicing defect observed in PPCs of two individuals with these mutations. Intriguingly, an AON that is complementary to c.4539+2001G>A rescued the splicing defect only in PPCs derived from an individual with STGD1 with this but not the other mutation, demonstrating the high specificity of AONs. In addition, a single AON molecule rescued splicing defects associated with different neighboring mutations, thereby providing new strategies for the treatment of persons with STGD1. As many genes associated with human genetic conditions are expressed in specific tissues and pre-mRNA splicing may also rely on organ-specific factors, our approach to investigate and treat splicing variants using differentiated cells derived from individuals with STGD1 can be applied to any tissue of interest.

Neutrophil (polymorphonuclear leukocytes [PMN]) transepithelial migration during inflammatory episodes involves INTRODUCTIONPolymorphonuclear leukocytes (PMN) are the first line of host defense against infection by bacterial pathogens and are rapidly recruited to sites of bacterial invasion. Because the majority of pathogens are encountered at mucosal surfaces, PMN must migrate out of the circulation, through the interstitium and across the epithelium to engage offending microbes. Although migration of PMN across the epithelium in this response is a terminal event, it is vitally important since elimination of pathogens and disease pathophysiology are direct consequences of PMN transepithelial migration. Despite the importance of this terminal event in the acute inflammatory response, many of the details regarding the regulation of PMN migration across mucosal surfaces remain undefined. Studies on this have revealed that migration of PMN across epithelial barriers involves a concerted series of cell-cell interactions between the PMN and epithelial cells (Zen and Parkos, 2003;Liu et al., 2004b). There is solid evidence that initial PMN-epithelial binding requires leukocyte ␤ 2 integrins, especially CD11b/CD18 (Parkos, 1997) and that the rate of PMN migration between epithelial cells is dependent on downstream signaling events from binding interactions between epithelial CD47 and PMNexpressed signal regulatory protein ␣ . Recent studies have begun to shed light on the nature of additional receptor-ligand pairs that may regulate PMN transepithelial migration in an organ-specific manner that are distinct from processes regulating transendothelial migration.Although the leukocyte ␤ 2 integrin CD11b/CD18 is a key adhesive element that regulates PMN transepithelial migration, there is evidence that additional adhesion molecules expressed on both PMN and epithelia must participate in PMN transepithelial migration, especially at the level of epithelial intercellular junctions. Recently, certain members of a growing family of proteins termed junctional adhesion molecules (JAMs) that are intercellular junction-associated, type-I Ig superfamily proteins (IgSFs) have been shown to serve as ligands for PMN and monocytes as they migrate across endothelial (Martin-Padura et al., 1998;Del Maschio et al., 1999;Johnson-Leger et al., 2002;Ostermann et al., 2002) and epithelial monolayers (Zen et al., 2004 (Ebnet et al., 2000;Takekuni et al., 2003) or desmosomes (Zen et al., 2004). JAMs are differentially expressed on a variety of endothelia, epithelia, and leukocytes and, under specific conditions, have been shown to mediate homophilic or heterophilic binding interactions that are important in regulating epithelial/endothelial monolayer barrier function and leukocyte transmigration (Martin-Padura et al., 1998;Cunningham et al., 2000;Liu et al., 2000;Cohen et al., 2001;Johnson-Leger et al., 2002;Liang et al., 2002;Mandell et al., 2004). In addition to homophilic/heterophilic interactions among JAM proteins, two family members, JAM-A ...

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